The present invention relates to a master timing generator and, more particularly, to a noise-immune master timing generator operable to produce timing signals synchronized with external reference signals and to maintain phase synchronization even in a noise environment. The noise-immune master timing generator is further operable to produce phase error signals in response to repeated minor phase error conditions or in response to major, more serious, phase error conditions.
In present day communication systems, especially those requiring a large number of precisely-timed operations, it is important that the timing of these operations be properly and correctly executed even in the presence of noise or other similar high-energy disturbances. By way of example, it is common in many digital communication systems to provide a master timing generator from which all timing signals for the system are derived and which are synchronized with an external reference signal to insure that all of the timing signals derived from the master timing generator are in proper, precise timing and phase relationship. If the external reference signal is corrupted by noise or other high-energy disturbances, the master timing generator will attempt to track the erroneous signal, thereby causing timing perturbations within the system and, consequently, causing serious synchronization problems within the system and degradation of the performance of the system. It is important in the above situation therefore that the presence of noise be quickly and readily detected so that the operation of the system can be altered during the presence of the noise or other appropriate measures taken to compensate for the presence of the noise. It may be further desirable in a system as described above to alter the timing of the master timing generator only when the reference signal is phase displaced with respect to the timing signals produced by the master timing generator by more than a predetermined specified minimum, for example, representing a "jitter" condition, thereby to prevent adjustment of the timing of the master timing generator for very minor changes in the timing of the reference signal. It may also be desirable in a system as described above to produce a phase error signal whenever the reference signal is repeatedly phase displaced with respect to the timing signals of the master timing generator by more than the predetermined minimum amount, and also to produce a phase error signal whenever the reference signal is phase displaced with respect to the timing signals of the master timing generator by more than a predetermined maximum, representing a serious phase error condition. These phase error signals may then be conveniently employed by external circuitry as a measure of the extent and frequency of the phase error conditions.
Apparatus commonly employed heretofore for correcting the timing of a master timing generator in a noise environment has generally been of a phase-locked loop type. However, this type of apparatus has a slow response time and is therefore slow to make corrections in the timing of a master timing generator and, in addition, will make slight phase adjustments when the reference signal is present in a noise environment or has very minor variations in timing, such as "jitter". Further, a phase-locked loop circuit does not normally indicate phase error conditions and, accordingly, does not normally provide a measure of the extent and frequency of the phase error conditions.